1. Field of the Invention
The present invention relates to an external fixator for use in orthopedic surgical applications, and more particularly to a method and apparatus for external fixation of large bones.
2. Discussion of the Related Art
In various orthopedic surgical procedures, it is often necessary to secure two bone portions in a relatively fixed relationship to each other. The need for establishing such a secured relationship is often a result of a fracture which has occurred to the bone. To ensure that the bone can regenerate in the proper orientation and fuse the fracture, it is important that the bone portions be fixed and in the desired position during bone regeneration.
Various devices are known for the fixation of bone during surgical procedures. For example, the various fixators manufactured by Orthofix S.r.l. for large bones include a bone screw mount which is connected by a ball joint to a central body which is telescopically extendable. The ball joint is able to secure the angularly position of the bone screw mount relative to the central body. The central body of the fixator has two components which are able to be telescopically displaced with respect to each other so as to selectively position the bone pins in the bone. A compression/distraction mechanism, which comprises a threaded screw, is temporarily placed between the central body and one of the bone screw mounts so as to adjust the amount of compression/distraction. When such a fixator is used, a template which resembled the fixator is initially placed in the region of the bone where the fixator is to be attached. A drill is then used to form the holes, and then the template is removed and the bone screws are installed. The bone screws are then attached to the fixator.
While the fixators of the type described above are effective, such fixators are nevertheless susceptible to improvements that may enhance the performance of the fixator. For example, the ball joint connection of the fixator requires a substantial torque to be applied to a cam mechanism so as to secure the ball joint in one particular orientation. In addition, a torque wrench is often necessary to ensure that the relatively large amount of torque required is placed on the ball joint. Accordingly, it would be desirable to reduce the amount of force required to secure the ball joint.
In addition, this type of fixator does not ensure that compression/distraction forces are in a direction parallel to the central axis of the bone. In other words, if the central axis of the fixator is not aligned with the axis of the bone, the force exerted intended to extend or contract the bone is not generally in an axial direction of the bone. Furthermore, the compression/distraction mechanism can bend when the fixator is expanded or compressed, and also can become disengaged from the fixator during operation. In addition, because only the central body expands, the amount to which the fixator is able to extend is somewhat limited (i.e., generally 4 cm of maximum expansion). Because the fixator required a large template be purchased to align the bone screws prior to attaching the fixator to the screws, the cost of using the fixator is also greater than desirable.
Finally, the use of the ball joints between the central body and the bone screw clamps of the fixator described above had several disadvantages. First, the ball joint allowed only 18 degrees of movement in each direction between the central body and the bone screw clamps. This may be considered limiting and it would be desirable to have a broader range of motion. In addition, the ball joint is retained in such a manner that the ball joint could become disassembled if the collar of the fixator and the central body are accidently rotated with respect to each other. Such disassembly would be clearly disadvantageous since this would happen typically when the fixator is being secured to the patient.